1. Field of the Invention
The present invention relates to a photo coupler in which a semiconductor laser and a semiconductor light-receiver are accommodated in a resin protector.
2. Description of the Related Art
A photo coupler including a semiconductor light-emitting device and a semiconductor light-receiver has been known. In such a photo coupler, when an electric input signal is inputted to the semiconductor light-emitting device, the semiconductor light-emitting device emits light corresponding to the input signal. Then, the semiconductor light-receiver receives and converts the light into an electric signal, and outputs the electric signal, whereby the signal is transmitted through the semiconductor light-emitting device and the semiconductor light-receiver.
In a general photo coupler, the semiconductor light-emitting device and the semiconductor light-receiver are embedded at a fixed interval in a resin protector, thereby electrically insulating the semiconductor light-emitting device and the semiconductor light-receiver from each other.
The signal is transmitted in a state where the semiconductor light-emitting device and the semiconductor light-receiver are electrically insulated from each other as described above, and accordingly, propagation of noise included in something except the signal can be prevented, and a withstand voltage of the photo coupler can be enhanced.
Heretofore, in the photo coupler as described above, a light-emitting diode has been applied as the semiconductor light-emitting device. However, since a large amount of carriers must be injected into the light-emitting diode in order to allow the light-emitting diode to emit the light, there has been a problem that it takes a long time to allow the light-emitting diode to emit the light. Therefore, it is difficult to apply the photo coupler including the light-emitting diode to high-speed communication in the order of Gbps (Giga bit per second), which has been required in recent years.
In this connection, a photo coupler in which a semiconductor laser having a current confinement structure and capable of emitting the light by being injected with a little amount of carriers is applied as the semiconductor light-emitting device has been used for the high-speed communication. However, a light irradiation range of the semiconductor laser is small, and accordingly, when the semiconductor light-receiver is disposed to be shifted from a predetermined position, there have been problems that the semiconductor light-emitting device cannot receive the light from the semiconductor laser, and that the semiconductor light-receiver cannot identify the signal even if the semiconductor light-receiver can receive the light therefrom.
In particular, in the case of a surface-emitting laser, a far field pattern (FFP) varies depending on a value of a current supplied thereto, but there is a light irradiation angle where intensity of the light is hardly changed even if such current value is changed. Accordingly, depending on the position where the semiconductor light-receiver is disposed, the semiconductor light-receiver sometimes receives the light with the same intensity even if the signal inputted to the surface-emitting laser varies. Therefore, there has been a problem that the semiconductor light-receiver outputs almost the same signals even if different signals are inputted to the surface-emitting laser.
A description will be specifically made of such problem in the surface-emitting laser with reference to FIG. 1. FIG. 1 is a graph showing relationships between the light intensities and the light irradiation angles for currents of 10 mA and 20 mA, which are supplied to the surface-emitting laser. Note that, in FIG. 1, an axis of ordinates represents the light intensity (mW), and an axis of abscissas represents the light irradiation angle (°). Here, the irradiation angle means an angle between 0° and a line that connects a predetermined position and a center of a light irradiation surface of the surface-emitting laser to each other when a position extended from the center of the light irradiation surface in a stacked direction of laser elements is defined as 0°. Moreover, a bold line in FIG. 1 indicates the light intensity when the current of 10 mA is supplied, and a thin line indicates the light intensity when the current of 20 mA is supplied. The semiconductor light-receiver is set capable of receiving light within an angle of approximately 1°.
In the case of a photo coupler in which the semiconductor light-receiver is disposed at a position along the irradiation direction (irradiation angle: 0°) of the surface-emitting laser, light in a range 100 corresponding to an area with irradiation angles from −0.5° to 0.5° is received by the semiconductor light-receiver. In the photo coupler with such configuration, a difference (refer to a shaded area 100a of FIG. 1) between a light quantity obtained by integrating the curve of 10 mA and a light quantity obtained by integrating the curve of 20 mA is large. Accordingly, two types of signals can be transmitted between the surface-emitting laser and the semiconductor light-receiver.
However, in the case of a photo coupler in which the semiconductor light-receiver is disposed at a position shifted by 8° from the irradiation direction of the surface-emitting laser, light in a range 101 corresponding to an area with irradiation angles from 7.5° to 8.5° is received by the semiconductor light-receiver. In the photo coupler with such configuration, there is hardly a difference (refer to an area 101a of FIG. 1) between the light quantity obtained by integrating the curve of 10 mA and the light quantity obtained by integrating the curve of 20 mA.
From this fact, the signals outputted in response to the received light quantities of two types of signals become almost the same, and accordingly, there has been a problem that the two different types of signals cannot be transmitted between the surface-emitting laser and the semiconductor light-receiver. In particular, the above-described problem has significantly appeared in a photo coupler including a small semiconductor light-receiver in which a diameter of a light receiving portion is approximately 100 μm, such as a pin-type photodiode.
In this connection, there is disclosed a technology for increasing the quantity of the light receivable by the semiconductor light-emitting device. In Patent Document 1 (Japanese Patent Publication No. 2001-358361), there is disclosed a photo coupler, in which the semiconductor light-emitting device and the semiconductor light-receiver are embedded in light-transmittable resin, and the light-transmittable resin is surrounded by light-shielding resin. In this photo coupler, the light is reflected on an interface of the light-transmittable resin has to the light-shielding resin, thus making it possible to prevent leakage of the light emitted by the semiconductor light-emitting device. Accordingly, the quantity of the light received by the semiconductor light-receiver can be increased to some extent. Therefore, transmission accuracy of the signals has been able to be enhanced.
However, in the above-described photo coupler according to Patent Document 1, only the leakage of the light emitted by the semiconductor light-emitting device and only noise light from the outside are prevented. Accordingly, the quantity of the light received by the semiconductor light-receiver is not increased very much. Therefore, there has been a problem that the accuracy at which two types of the signals are transmitted is insufficient.